In the field of electrophotographic photoreceptors, there has recently been an increasing demand to extend the photosensitive wavelength region of conventional organic photoconductive materials to a wavelength region of a semiconductor laser in the near infrared light region (780 to 830 nm) so as to make them applicable to a digital recording system, such as a laser printer. From this point of view, there have been reported photoconductive materials for semiconductor lasers, such as squarylium compounds as disclosed in JP-A-49-105536 and JP-A-58-21416, triphenylamine type tris-azo compounds as disclosed in JP-A-61-151659, and phthalocyanine compounds as disclosed in JP-A-48-34189 and JP-A-57-148745 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In cases where an organic photoconductive material is used as a photosensitive material for semiconductor lasers, it is required to have a photosensitive wavelength region extended to a longer side and to provide a photoreceptor having satisfactory sensitivity and durability. None of the above-described conventional organic photoconductive materials sufficiently satisfies these requirements.
In order to overcome the drawbacks of the conventional organic photoconductive materials, the relationship between their crystal form and electrophotographic characteristics has been studied. In particular, many reports have hitherto been made on phthalocyanine compounds.
It is known that phthalocyanine compounds generally exhibit several different crystal forms depending on the process of synthesis or the process of treatment and that the difference in crystal form has a great influence on their photoelectric conversion characteristics. For example, known crystal forms of copper phthalocyanine compounds include .alpha.-, .pi.-, .chi.-, .rho.-, .gamma.-, and .delta.-forms as well as a stable .beta.-form. These crystal forms are known capable of interconversion by application of a mechanical strain, sulfuric acid treatment, organic solvent treatment, heat treatment, and the like as described, e.g., in U.S. Pat. Nos. 2,770,629, 3,160,635, 3,708,292, and 3,357,989. JP-A-50-38543 refers to the relationship between a crystal form of copper phthalocyanine and its electrophotographic characteristics. As for gallium phthalocyanine crystal forms, JP-A-1-221459 describes two crystal forms obtained by acid pasting. Further, the inventors of the present invention previously revealed that five crystal forms of hydroxygallium phthalocyanine exhibit excellent electrophotographic characteristics (see JP-A-5-263007).
For the production of these crystals, a so-called acid pasting process as described in Bull. Soc. Chim., 23, France (1962) is adopted. That is, a starting gallium phthalocyanine compound is subjected to acid pasting to once obtain metastable hydroxygallium phthalocyanine, which is then subjected to a solvent treatment for transformation.
Starting gallium phthalocyanine to be used in acid pasting includes chlorogallium phthalocyanine (see D.C.R. Acad. Sci., Vol. 242, p. 1026 (1956), JP-B-3-30854 (the term "JP-B" as used herein means an "examined published Japanese patent application"), JP-A-1-221459, and Inorg. Chem., Vol. 19, p. 3131 (1980)), bromogallium phthalocyanine (see JP-A-59-133551), and iodogallium phthalocyanine (see JP-A-60-59354).
However, hydroxygallium phthalocyanine having a specific crystal form which is prepared by usual acid pasting, even with the crystal form being equal, show variation in performance as a electrophotographic photoreceptor, such as sensitivity and a dark decay rate, and it has been difficult to obtain hydroxygallium phthalocyanine which provides stable image characteristics.